Plant Community Structure, Fire Disturbance, and Recovery in ...

Contributions to the Study of Biological Diversity Vol. 3
Peninsula is capable of forming a Tropical Moist
Forest. Because of its location near the equator,
Guyana’s climate is dominated by the equatorial
trough and the seasonal movements of the
intertropical convergence (ITC) (Snow 1976).
The mean annual temperature for the coastal
zone of Guyana is approximately 26.5ºC, with
a 1ºC to 1.5ºC range between the mean
temperatures of the warmest and coolest months,
which coincide with the dry and wet seasons
respectively. The trade winds consistently blow
from the east-northeast or the east, with monthly
mean speeds varying from 3 to 4 m/second.
Precipitation patterns are characterized by two
dry seasons and two wet seasons per year. The
major wet season runs from April to August,
with a minor wet season from November to
February. Mean annual rainfall in the coastal
area of the Northwest District has been reported
from approximately 2,500 mm per year (Snow
1976) to 3,400 mm per year (K. Richardson,
pers. comm.). The wettest month is sometimes
reported as June, with an mean rainfall of 400
mm (Snow 1976), or January with a mean
rainfall of 500 mm (K. Richardson, pers.
comm.). During a typical year in the stronger
dry season of February-April, only 5 to 10 days
per month receive rainfall (Snow 1976).
Snow (1976) noted that a zone of minimum
rainfall probably occurs along a narrow band of
the coast of northern South America, with a peak
in precipitation reached less than 50 kilometers
inland, caused by effects of increased surface
roughness of forests upon air leaving the ocean.
This has been confirmed by many observations
(Hollowell, unpublished) of rain developing
several kilometers inland from the Waini
Peninsula while extreme coastal areas remained
dry. The typically low number of rain days in
the dry season, combined with reduced rainfall
immediately along the coast and a drought
caused by a strong El Niño event, led to
extremely dry conditions in the coastal forests
of the Waini Peninsula in 1997-1998.
Geomorphology
The coastal plain sediments of the Guianas
were deposited in recent times on the margins
of the Guiana Shield, which is a natural
geological unit characterized by ancient bedrock
with sandstone and igneous highlands (Gibbs
11
& Barron 1993; Huber 1995a). The Guiana
Shield is geologically distinct from the more
recently uplifted Andean highlands. It is
bounded for the most part by the Atlantic Ocean
to the east, the Orinoco River to the north and
west, and the Negro and Amazon Rivers to the
South. The Shield is widely considered to be a
region of high biotic endemicity (Huber 1995c),
possibly due to having served as a refugium for
tropical forest species during prolonged periods
of dry climate in the Pleistocene epoch (1.8 mya
to 11,000 years bp), or due to the ancient, varied,
and isolated landscapes of the region (Colinvaux
1987).
From the Orinoco Delta south to the
Amazon River, the coast of South America is
geologically referred to as a “passive margin,”
with deep sediments deposited over the
tectonically stable Guiana Shield (Di Croce et
al. 1999). That stability is reflected in estimates
that relative sea level in the region has been
relatively stationary since post-glacial rises
ended perhaps 6,000 B.P., although sea levels
possibly surpassed that of the present by as much
as five meters from 6,000 to 3,000 B.P. (Pirazzoli
1991). By some accounts, the section of the
Guyana coast including the Waini Peninsula has
been subject to some uplift in recent geological
times (Gibbs & Barron 1993), while others
suggest that subsidence may have been a factor
along the northwestern coast of Guyana
(Brinkman & Pons 1968). Williams (1989)
employed archaeological evidence to infer that
subsidence in the Guyana’s northwest ended
around 3,400 years B.P. The presence of marine
sediments on higher elevations in the central
parts of the Waini Peninsula suggests that in
geologically recent times, relative sea levels
may have once been several meters higher than
present or that moderate uplift has occurred. The
area is close enough to a geologically active area
beginning just south of Trinidad (Di Croce et
al. 1999) that associated minor uplift or
subsidence around the study site could be
plausible.
The Shield’s basement granites are
estimated to be as far as 500 meters below the
surface along Guyana’s northwest coast (Di
Croce et al. 1999). The coast of the Guianas is,
in essence, the major portion of the coastal delta
of the Amazon River. The Essequibo, Waini, and